A coolant composition introduced into the cooling system for a diesel engine is intended to reproducibly carry heat from heat generating components within the engine to a cooling portion within the system, and then to be recycled continuously through the engine during operation. The coolant composition optimally is not affected by the high temperature encountered within the cooling system, nor is it chemically modified as a result of being cycled through the system. Further, the coolant composition assists in protecting metal parts from corrosion effects which would be encountered in the cooling system environment when the engine is not operating.
The coolant composition thus has to carry out a number of functions within the cooling system to perform satisfactorily. It must have a sufficiently high thermal conductivity to be able to absorb heat, but be sufficiently stable so as to not decompose or otherwise change in chemical composition during engine operation. Further, because the interior of the cooling system contains metal components of differing compositions such as iron, brass, solder, aluminum, and steel, the coolant must be both unaffected by the metals and able to protect the metals in this environment from degradation. Further, the coolant composition must be able to provide its heat absorbing and protective function over an extended period of time.
Glycols have long been used in coolant compositions as the primary coolant and freezing point depressant component. Depending on the temperature range of the internal environment in which the engine is to be operated, varying amounts of water may typically be added to extend the coolant composition. As a result the user is able to dilute the coolant composition as needed to obtain the desired low and high temperature protection under the expected duty conditions. It should be noted, though, that the protection against corrosion of the internal parts of the cooling system is required over the entire range of dilution of the coolant composition.
Certain of the components in the coolant composition may also require concentration adjustment, composition adjustment, or both, in response to the nature and quantity of the component metals used in the coolant system. Thus, engines which incorporate higher levels of aluminum-containing parts should utilize coolant compositions providing corrosion protection for aluminum components. Alternatively, where the cooling system incorporates primarily or exclusively iron and iron-containing metals, the corrosion protection components may differ in concentration, composition, or both, to optimize the coolant composition over its duty cycle for use with these metals.
In the case of cast iron cooling systems typically used in connection with diesel engines, pitting of the metal surface in contact with the coolant composition is a concern. In wet sleeved diesel engines, pitting of the engine cylinder liners is a primary concern. These engine liners are made of cast iron, and traditionally rely on the presence of sodium nitrite in the coolant composition to prevent or minimize pitting of the liner surface. In a coolant composition for use with diesel engines, the sodium nitrite is consumed as it protects the iron surfaces, often being oxidized to sodium nitrate. In addition, the sodium nitrite in the coolant composition tends to act aggressively on any aluminum or solder surfaces within the cooling system. As a result, the concentration of nitrite in the coolant composition must be carefully controlled to balance the protective effect on iron surfaces with the corrosive effects on the other system components. To evaluate if the nitrite concentration remains within defined limits for a particular coolant system application, coolant can be applied to commercially-available test strips. Representative test strip products are available as Acustrip® CTS-3 from Acustrip, P.O. Box 413, Mountain Lakes, N.J. 07046 or as Fleetguard® Coolant Test Strips from Cummins Filtration, 1200 Fleetguard Road, Cookeville, Tenn. 38506.
Even where the nitrite is properly balanced, depletion of this component is a function of time and temperature which typically varies with service life and the duty cycle of the engine. Though other components are introduced into the coolant composition to eliminate or minimize the corrosion effect on the various metal surfaces in the cooling system, the sodium nitrite component is generally unique in its capacity to control pitting on iron surfaces in the cooling system. As such, depletion of nitrite in the coolant composition can compromise the ability of the coolant composition to protect against corrosion.